C-Fortran interface

I do not have experience of interfacing Fortran with C.. I mean exchanging
and sharing data between C routines and Fortran routines linked together.
I would appreciate some guidelines how to do that.

Basically, assume the following common block is defined in a fortran
routine called "m1505.f":
COMMON/LEXP/DLEXP,TO,AINT

That is the Fortran routine defined the common block named "LEXP"
containing the three valiables DLEXP, TO, AINT
Now assume that such a Fortran routine is linked to some C routines and
some Tcl/Tk procedures through a GNUmakefile.
How can I, from inside a C routine, access the content of the variable
AINT in the common block LEXP ?